The present invention relates to the field of multi-layer circuit boards; more specifically, it relates to multi-layer hybrid surface mount technology (SMT)/pin in hole (PIH) circuit board and a method for soldering a multi-layer hybrid SMT/PIH connector to the board.
During assembly of a hybrid SMT/PIH connector to a hybrid SMT/PIH circuit printed circuit board (PCB), two goals must be accomplished. First the SMT connects on the connector must be soldered to the SMT pads on the PCB and second, the pins on the connector must be soldered in the PIHs on the PCB. During rework of a hybrid SMT/PIH printed circuit board (PCB), specifically in the process of removing a hybrid SMT/PIH connector from the board, three goals must be accomplished. First, the proper degree of heat must be maintained to allow the solder securing the SMT connects to the PCB to reach and remain at the solder reflow temperature. Second, the proper degree of heat must be maintained to allow the solder securing the pin connects to the PCB to reach and remain at the solder reflow temperature. Third, it is necessary to prevent the heat applied, especially to the pin connects, from dispersing through the PCB to adjacent components and causing damage to the PCB and components attached thereto. Note that whenever the term PCB is used the more generic term circuit board (CB) may be substituted.
It is common to mount connectors (or active components) to PCBs by inserting pins extending from connectors into plated through holes (PTHs) in the PCB and wave soldering them in place. The PTHs normally provide connections between the pins and conductive planes. Conductive planes may be power planes or signal planes containing signal lines situated at various levels of the PCB. In the case of a connector, the PTHs provide connection to power planes in the PCB. It is also common to mount connectors (or active components) to PCBs using SMT technology using solder paste applied to the SMT pads on the PCB. It is the combination of the two interconnect technologies, SMT and PIH that presents soldering and de-soldering challenges. Hybrid SMT/PIH connectors have hereto been soldered using solder paste applied to the SMT pads as well as filling the PTHs with solder paste.
Particularly challenging is the case where pins must be soldered to PTHs connected to different numbers of power planes in a multi-layer PCB. Heat is normally conducted away from the PTH by the power planes inhibiting the solder from reaching the solder reflow temperature. The more power planes connected to a PTH the more heat is conducted away. Thus, some pins become hotter than other pins during soldering (or some do not become hot enough), and some pins remain cooler than other pins (or some pins become too hot, damaging the PCB) during de-soldering.
Turning to the figures, FIG. 1 is a partial top view of a related art hybrid SMT/PIH circuit board. In FIG, 1, PCB 100 is a hybrid SMT/PIN PCB and includes a plurality of SMT pads 105 disposed on a top surface 106 of the PCB. Also formed in PCB 100 are a plurality of PTHs. Three PTHs 110A, 110B and 110C are illustrated. PTH 110A includes a plated barrel 115A and a lip 120A. PTH 110B includes a plated barrel 115B and a lip 120B. PTH 110C includes a plated barrel 115C and a lip 120C. SMT pads 105 and lips 120A, 120B and 120C may be formed by subtractive etching of a copper layer formed on top surface 106 of PCB 100. Plated barrels 115A, 115B and 115C may be formed by drilling through PCB board 100 and plating the sidewalls of the resulting hole.
FIG. 2 is a partial cross-sectional view of the hybrid SMT/PIH circuit board through line 2xe2x80x942 of FIG. 1. As may be seen in FIG. 2, each of PTHs 110A, 110B and 110C extends from top surface 106 to a bottom surface 112 of PCB 100. In FIG. 2, PCB board 100 is comprised of a plurality of (in this example, twelve) power planes 121 through 132. Plated barrel 115A of PTH 110A is connected to one power plane, power plane 123. Plated barrel 115B of PTH 110B is connected to three power planes; power planes 122, 125 and 129. Plated barrel 115C of PTH 110C is connected to six power planes; power planes 121, 124, 126, 127, 130 and 132. A plurality of signal lines 150 are dispersed between power planes 121 through 132 in a plurality of signal planes 155. Signal planes 155 are separated from power planes 121 through 132 by dielectric material. In one example the xe2x80x9csandwichxe2x80x9d structure of PCB 100 may be formed by etching a signal plane on one side and a power plane on the other side of a printed circuit board joining multiple boards with epoxy or other dielectric adhesive, wherein the dielectric material is epoxy/glass material, fluropolymer, allyated polyphenyl esters, cyanate ester epoxy,( i.e. epoxy, PTFE, or other known dielectric) on one side of signal lines 150 and epoxy or other dielectric adhesive known in the art on the other side of the signal lines.
Also illustrated in FIG. 2 is a connecter 160. Connector 160 is a hybrid SMT/PIH connector and includes a plurality of SMT connectors 165 and a plurality (in this example, three) of pins 170A, 170B and 170C. A plurality of SMT connectors 165 are attached to SMT pads 105 by SMT solder joints 175 and pins 170A, 170B and 170C are attached respectively to PTHs 110A, 110B, and 110C by PTH solder joints 180A, 180B and 180C respectively.
FIG. 3 is a partial side view of the hybrid SMT/PIH circuit board of FIG. 1. In FIG. 3, connector 160, one pair of SMT connectors 165 and one pin 170 are illustrated. SMT connector 165 is attached to SMT pad 105 by SMT solder joint 175 and pin 170 is attached to plated barrel 115 by PTH solder joint 180.
FIG. 4 is a partial view of power plane 123 through line 4xe2x80x944 of FIG. 2. SMT pads 105 are shown by dashed lines for reference purposes only and are obviously not present in power plane 123. In FIG. 4, plated barrel 115A of PTH 110A is co-extensive with power plane 123. Each plated barrel 115B and 115C of PTH holes 110B and 110C are separated from power plane 123 by a gap 185. Gaps 185 are filled with epoxy or other dielectric adhesive during the assembly pf PCB 100.
A first aspect of the present invention is a multi-layer circuit board, comprising: a plurality of conductive planes; a plurality of plated through hole sets, each set comprising one or more plated through holes, none to all of the plated through holes of each set contacting at least one conductive plane; a thermal break formed around each plated through hole in each conductive plane to which the plated through hole is connected; and one or more thermal vents, in the vicinity of each plated through hole in each conductive plane to which the plated through hole is connected.
A second aspect of the present invention is s method of fabricating a multi-layer circuit board, comprising: forming a plurality of conductive planes; forming a plurality of plated through hole sets in at least one of the conductive plane, each set comprising one or more plated through holes, none to all of the plated through holes of each set contacting at least one conductive plane; forming a thermal break around each plated through hole in each conductive plane to which the plated through hole is connected; forming one or more thermal vents, in the vicinity of each plated through hole in each conductive plane to which the plated through hole is connected; placing the conductive planes on top of each other with a dielectric layer located between the conductive planes; and joining the conductive planes together to form the multi-layer circuit board.
A third aspect of the present invention is A method of attaching a hybrid pin in hole/surface mount technology connector or component to a multi-layer circuit board, comprising: providing the hybrid pin in hole/surface mount technology connector or component comprising a body having a plurality of pins and surface mount connectors mounted thereon; providing the multi-layer circuit board comprising; a plurality of conductive planes; a plurality of surface mount technology pads formed on a top surface of the multi-layer circuit board; a plurality of plated through hole sets, each set comprising one or more plated through holes, none to all of the plated through holes of each set contacting at least one the conductive plane; a thermal break formed around each plated through hole in each conductive plane to which the plated through hole is connected; and one or more thermal vents, in the vicinity of each plated through hole in each conductive plane to which the plated through hole is connected; screening solder paste on to the surface mount technology pads; inserting the hybrid pin in hole/surface mount technology connector or component into the multi-layer circuit board; reflowing the solder paste to solder the surface mount connectors to the surface mount pads; and wave soldering the pins to the plated through holes from a bottom surface of the multi-layer board.
A fourth aspect of the present invention is a multi-layer circuit board comprising: first and second plated through holes, each adapted to having a pin positioned therein; the first and second plated through holes each connected to a different number of conductive planes contained within the circuit board; and the conductive planes having thermal breaks and thermal vents positioned around the plated through holes in each of the planes to which each plated through hole is connected, the thermal breaks assuring sufficient heat retention within each plated through hole to allow formation of a solder joint between the pins and the plated through holes, the pins being at different temperatures during formation of the solder joint.